1. Field of the Invention
The present invention relates to fiber reinforced prepregs containing a thermoplastic (methacrylate butadiene styrene) modified thermoset resin and to molded articles prepared from such prepregs.
2. Description of the Prior Art
Electrical and office equipment enclosures, such as computer cases, copier cases and telecommunications equipment, have previously been prepared from thermoplastic resins such as polycarbonates, acrylonitrile butadiene sytrene (ABS) and polypropylene. These materials have the advantageous properties of toughness, flexibility and the ability to meet UL specifications by including fire retardant additives. However, the thermoplastics have the disadvantages of not being stiff due to their low modulus and an inability to flow into detailed molds due to their inherently high (molecular weight) melt viscosity. The high melt viscosity also results in an inability to be compounded with much more than 10 to 40% chopped fiber reinforcement resulting in less than half the modulus values obtainable with thermosets even when such compounded thermoplastics are used. Also because of the relatively high melt viscosities and lower modulus, thicker wall sections and, in some cases, ribs must be designed into molded parts to provide adequate rigidity and help to prevent burn (melt) through during flammability testing. As a practical matter, it is exceedingly difficult to mold wall sections much less than 0.080" thick due to the thermoplastic resins high melt viscosity and subsequent inability to flow into wall cavities this narrow with conventional injection molding equipment. The thermoplastics also exhibit the phenomena of melting when heat is applied. This phenomena has required the flammability test UL 94 5 V itself (burn through test) to be modified so that the gas flame source is angled 20 degrees under the specimen to prevent melted material from dripping into the flame.
One way to overcome the disadvantages of melting (distortion) and the need for thick wall designs due to low modulus would be to use a thermoset resin. However, the conventional thermoset resins (i.e, epoxies and phenolics) are not tough enough for these applications resulting in cracking due to their relatively low impact strength.